An optical memory technology using optical disks as high density, high capacity storage media with pit-shaped patterns, is widening its applicability to digital audio disks, video disks, document file disks and data files, etc.
According to this optical memory technology, information is recorded onto and reproduced from an optical disk via micro-focused light beams with high accuracy and reliability. This writing/reading operation solely depends on its optical system.
The basic functions of the optical head, which is a major component of the optical system, are broadly grouped under convergence that forms micro spots of diffraction limits, focus control and tracking control of the above-described optical system, and detection of pit signals. These functions are provided by combining various optical systems and photoelectric transfer detection systems according to the purpose and application.
On the other hand, a high density, high capacity optical disk called “DVD” is put to practical use and stepping into a limelight as an information medium capable of handling large capacity information such as moving images in recent years. Compared to a compact disk (hereinafter, abbreviated as “CD”), which is a conventional optical disk, this DVD optical disk reduces a pit size on the information recording surface to increase recording density.
Thus, the optical head for writing/reading a DVD optical disk is different from that for a CD in a wavelength of a light source which determines a spot diameter and numerical aperture (hereinafter abbreviated as “NA”) of a converging lens. By the way, while the wavelength of the light source of the CD is 0.78 μm and NA is approximately 0.45, the wavelength of the light source of the DVD optical disk is approximately 0.63 μm to 0.65 μm and NA is approximately 0.6.
Thus, writing/reading two types of optical disks, CD and DVD optical disks using a single optical disk drive requires an optical head provided with two optical systems.
On the other hand, in order to meet requirements of miniaturization, slimming and cost reduction of the optical head, the CD and DVD optical systems tend to share as many parts as possible with each other. For example, a method that two types of converging lens may be used for the DVD optical disk and for CD separately using the light source for DVD for both systems, a method that a converging lens may also be shared by making NA mechanically or optically changeable to provide greater NA for the DVD optical disk and smaller NA for the CD, or the like is adopted.
For example, as disclosed in Japanese Patent Laid-Open No. 9-219035, a method of providing compatibility between DVD and CD by optimizing a part of a converging lens which is optimized for DVD to the thickness of a CD base material in ring zone form is proposed. Of the above-described optical heads, the above-described method will be explained below with reference to the attached drawings.
FIG. 8 shows a configuration of an optical system of the optical head disclosed in Japanese Patent Laid-Open No. 9-219035. In the case of a general optical apparatus as illustrated here, an objective lens 23 is provided on an optical path between a disk 7 and optical detector 4 and a light source 1 is placed on an optical path branched from a beam splitter 5. In the optical apparatus of the above-described invention, the objective lens 23 has a characteristic shape as shown in FIGS. 9 (a) and (b).
A special part (part which is different from a normal lens and is an intermediate area separating a peripheral area which will be described later from a central area) is provided on at least one of the plane of incidence or plane of outgoing radiation of the objective lens 23. This special part is a doughnut-shaped or ring-shaped intermediate area A2 having a smaller outer diameter than the overall effective diameter of the optical path area. A central area A1 is provided inside the intermediate area A2 and a peripheral area A3 is provided outside the intermediate area A2. The curvature of the above-described central area A1 and peripheral area A3 is optimized with respect to a thin DVD (digital video disk) 7a and the curvature of the above-described central area A2 is optimized with respect to a thick CD7b (compact disk).
Then, it is desirable that the above-described optical detector 4 be designed so that the light from the peripheral area corresponding to the far-axis area does not reach when information is reproduced from a thick disk, that is, the light reaches only the central area A1 and intermediate area A2 of the objective lens. Therefore, as indicated with dotted lines in FIG. 9 (b) when the thick CD7b is written or read, the light in the central area A1 and intermediate area A2 converges to the CD7b. In this case, even if the curvature of the central area A1 corresponding to the near-axis area is optimized to the thin DVD7a, the near-axis light close to the central axis of the lens passes, which causes less spherical aberration.
Then, when the DVD7a is written or read, the light passes through the central area A1 and peripheral area A3 having the curvature optimized to the thin disk to form a focus on the information surface of the thin disk 7a. 
If the numerical aperture of the areas corresponding to the central area and intermediate area of the above-described objective lens 23 is reduced to 0.4 or smaller, it is also possible to form a small spot for a thick disk and thereby form a spot of a size optimized to the CD disk.
However, when a CD optical disk is written or read, the above-described conventional configuration has a problem that jitter (a value indicating variation of the time axis) increases considerably depending on the phase position of the central area. That is, the problem is that because of lens manufacturing errors, constraints or improvement of the performance except jitter (e.g., degree of matching between the 0 level of a focus error signal and minimum jitter focus position), it is difficult to secure an appropriate jitter value when the phase position of the above-described intermediate area changes.
Furthermore, in the case of an objective lens provided with a ring zone of the conventional special section, there may be considerable level differences at the edge of the ring zone and it is difficult to perform molding using a glass press lens with high temperature stability. For this reason, molding is performed solely using a plastic lens and it is therefore necessary to create the lens and design the optical head taking into account aberration variations with temperature.
These problems will be explained with reference to the attached drawings below.
FIG. 10 (a) is a graph illustrating wavefront aberration during a CD read when NA of the objective lens 23 is 0.6 and NA inside the intermediate area A2 is 0.39 and FIG. 10 (b) is a graph illustrating a relationship between the phase difference of the intermediate area A2 with respect to the central area A1 and CD read jitter calculated using a simulation. The phase of the central area A2 is based on the phase of the central part of the objective lens 23 and the direction in which the phase of the intermediate area A2 delays from the phase of the central part is assumed to be positive.
As apparent from FIG. 10, when the thickness of the corresponding base material of the intermediate area A2 is 1.2 mm corresponding to the thickness of the CD base material, that is, when the intermediate area A2 is set so that its spherical aberration is optimal for the 1.2 mm CD, jitter may deteriorate drastically depending on the phase. That is, this indicates that, when the phase deviates from an ideal state because of manufacturing errors, etc., there is a problem that it may be difficult to secure CD read jitter.
Moreover, the objective lens with the conventional central ring zone may have considerable level differences in both of or at least one of the inside and outside perimeters of the ring zone and even if an attempt is made to perform molding using a glass lens with high temperature stability, the molding is difficult from the standpoint of volume production or even if the molding is applicable to the shape, a large transition area (area in which the shape of the section including level differences inevitably differs from the ideal shape from the standpoint of manufacturing of the molding die and could cause deterioration of a read signal, etc.), causing a problem that it is impossible to obtain sufficient characteristics.